The present invention relates to a machine for checking the weight of objects in continuous movement; these machines are known generally as check-weighing machines.
Such machines comprise a belt conveyor which totally acts on a balance arranged to measure the weight of objects in transit on said belt conveyor. This conveyor is composed essentially of two rollers, one of which is a drive roller, an endless flexible belt stretched between the two rollers, a fixed support structure comprising a fixed slide table for the upper branch of the belt, and a geared motor unit which drives the drive roller for the belt. The weight of all these members acts on the balance, the upper branch of the belt basically defining the weighing plate.
Two further belt conveyors are generally provided upstream and downstream of said weighing belt respectively; these conveyors do not act on the balance and have a linear speed substantially equal to that of the weighing belt.
The purpose of the upstream belt conveyor is to withdraw the objects from an upstream conveying line and lead them onto an intermediate weighing belt; its presence is necessary to obtain correct spacing of the objects and to give them the same speed as the weighing belt to prevent them from undergoing dynamic influence; any speed variation between the weighing belt and the upstream belt would, in fact, harm the weighing accuracy.
The purpose of the downstream conveyor is merely to lead the weighed objects away and possibly to support any devices for removing or expelling those objects which have been found to be outside the weight tolerance. Its presence is relatively less necessary than the upstream belt.
Measurement precision is extremely important in these machines, and even more so the smaller the weight of the objects. Consequently with the passing of time the construction of these machines has been continuously improved by placing special emphasis on reducing, as much as possible, the influence of the weighing belt on the balance measurements; in this respect this represents a relatively high tare value, and it has therefore been sought to reduce its weight and in particular the weight of the geared motor unit to possible values, and in addition it has been sought to prevent vibration and dynamic influence as much as possible.
Although known machines operate satisfactorily, they are however susceptible to improvement with regard to weighing accuracy and response.
As is well known, the accuracy of a balance is proportional to the quality of its construction and to the size of the relative range of measurement, while the weighing response depends on the inertia of the masses in play.
In known machines, by far the largest weight acting on the balance is that of the geared motor unit which drives the weighing conveyor; in addition both the motor and the reduction gear are sources of vibration which are disturbing because of their frequency and even more so because of their dynamic intensity, in that they originate from considerable masses.
With regard to accuracy, it will be apparent that in checking objects weighing just a few grams, the balance is loaded by a conveyor and the relative geared motor unit which together weight some hundreds of grams. The balance may thus be subjected, for example, to a tare of 1000 g (the weight of the conveyor belt plus geared motor unit) plus 20 g representing the weight of the object. In seeking to obtain an accuracy of at least 1%, the error in absolute terms has to be limited to within two tenths of a gram in a total weight of more than 1 kg. Thus in relative terms the accuracy must thus be just a few parts in 10,000.